1. Field of the Invention
The present invention relates to radiation imaging apparatuses for medical diagnoses or industrial nondestructive inspections and, more particularly, to a radiation imaging apparatus and a radiation imaging system suitable for taking moving pictures, where the radiation includes not only X-rays but also alpha-rays, beta-rays, and gamma-rays.
2. Description of the Related Art
Hitherto, X-ray imaging systems installed in hospitals or the like adopt two imaging technologies. A film imaging technology in which a patient is irradiated with X-rays and a film is exposed to the X-rays transmitted through the patient, and a digital imaging technology in which X-rays transmitted through a patient are converted into electrical signals, which are detected as digital values by an analog-to-digital converter to store the detected digital values in a memory. In the latter technology, a visible light emitted from a photostimulable phosphor, that is called an imaging plate (IP) mainly made of BaFBr:Eu, is converted into electrical signals by a photomultiplier for digitization by temporarily storing X-ray images in the IP and, then scanning the IP with laser beams.
Recently, a technology has been put into practical use in which an X-ray to visible-light converting phosphor mainly made of Gd2O2S:Tb or CsI:TI, is irradiated with X-rays and visible light emitted in proportion to the amount of the X-rays is converted into electrical signals by an amorphous silicon light sensor for digitization. Apparatuses adopting this technology are called flat panel detectors (FPDs). One type of the FPDs, which is made of Se or PbI2, directly absorbs X-rays and converts the absorbed X-rays into electrical signals, without using the X-ray to visible-light converting phosphor.
In another apparatus, a primary phosphor is irradiated with X-rays, photoelectrons emitted from the screen of the primary phosphor are accelerated and converged by using an electron lens, and the X-ray images on a secondary phosphor are converted into electrical signals by using an image pickup tube or a charge coupled device (CCD). Such an apparatus is called an image intensifier (II), which is a common technique for use in fluoroscopy. The image intensifier is one of the digital imaging techniques which can detect electrical signals as digital values.
As described above, there are various technologies for digitalizing X-ray images.
Digitalization has been increasingly required in the medical field in recent years. The digitalization of image data advantageously facilitates recording, displaying, printing, and storing of radiographed data. Image-processing the radiographed data by using a computer can support diagnosis by a doctor. Furthermore, automatic diagnosis by using only a computer without the intervention of a doctor can be realized in the near future.
Even in the medical field of the process of moving from film imaging technology, that is, an analog imaging technology, to the digital imaging technology described above, the first step of radiography is plain radiography. Plain radiography is called plain chest radiography for, for example, a chest, in which a human body is radiographed from the front (or a side) of the chest. It is said that a half size (35 cm×43 cm) or more or, if possible, a size larger than 43 cm×43 cm is generally required as an imaging area in order to cover the entire chest (the upper body) of a human body. The FPD technology is more promising than the II technology which has distorted peripheral images in the plain chest radiography.
Because body information concerning a region, such as an esophagus, trachea, lung blood vessel, alveolus, heart, cardiovascular, diaphragm, rib, or clavicle, in the neighborhood of the lung field in the upper body can be radiographed on one sheet by the plain chest radiography, the plain chest radiography is frequently adopted as a useful technology for screening focus. However, because transmitted images are observed in the plain chest radiography, it can be difficult to detect the shadow of focus that is overlapped in the transmitted images when the focus to be observed exists, for example, behind a rib or diaphragm or in the shadow of a cardiovascular portion. Accordingly, there is a problem that the efficiency of focus screening is decreased and the detection of focus can be delayed.
In order to solve such a problem, a method is realized in which radiography is performed two times by using two imaging plates (IPs) with the X-ray tube voltage being varied and subtraction is performed for X-ray images on the two IPs to remove the shadow of bones. This method, which is called energy subtraction (ES), utilizes the fact that bone tissue differs in absorptivity of X-ray energy from soft tissue, such as a blood vessel, lymphatic, or nerve, when the X-ray energy is varied.
Examples of energy subtraction will now be described. Japanese Patent Laid-Open No. 2-273873 discloses a radiographic method in which subtraction is performed after distortion is corrected in images that have been radiographed with radiation emitted from a plurality of radiation sources having different energy levels based on the image signals. Japanese Patent Laid-Open No. 3-106343 discloses a structure in which X-rays having different energy levels are generated, simultaneously with the acquisition of images, by a dual energy generating mechanism that is provided at an X-ray irradiation hole of an X-ray tube. Japanese Patent Laid-Open No. 3-133276 discloses a method for displaying energy-subtracted pictures, in which the pictures of only diseased tissue acquired as difference signals are added as three-dimensional depth information for display. Japanese Patent Laid-Open No. 5-260382 discloses a structure in which images radiographed with X-rays having different energy levels are recorded in different parts in one fluorescent sheet and subtraction is performed for the images. Japanese Patent Laid-Open No. 2000-116637 discloses a structure in which a fluoroscopic actual image of an object and a reference image are displayed in a common display at a different moment.
Although energy subtraction is useful for removing the shadows of bones, there is no guarantee that the shadows of the bones are entirely removed. Particularly, a part of the shadows of bones is disadvantageously left depending on the body type or the physical constitution of a patient or on the kind of focus. For example, focus does not always exist in the shadow of a rib and, therefore, it is not sufficient to perform only energy subtraction for removing the shadows of bones depending on the state (physical constitution or focus) of a patient when the focus exists in the shadow of a heart or diaphragm. In addition, it is difficult to detect focus when either still images or moving pictures are observed. Particularly, if the motion in a human body is relatively slow in the moving pictures, it is difficult to detect focus because of a small variation in the moving pictures. Furthermore, with the structure disclosed in Japanese Patent Laid-Open No. 2000-116637, there is a problem that it is difficult to compare the real image with a reference image because the real image and the reference image are displayed in a common display at a different moment.